TLS M. Shore
Internet-Draft No Mountain Software
Intended status: Standards Track R. Barnes
Expires: December 28, 2015 Mozilla
S. Huque
Verisign Labs
June 26, 2015
A DANE Record and DNSSEC Authentication Chain Extension for TLS
draft-shore-tls-dnssec-chain-extension-00
Abstract
This draft describes a new TLS extension for transport of a DNS
record serialized with the DNSSEC signatures needed to authenticate
that record. The intent of this proposal is to allow TLS clients to
perform DANE authentication of a TLS server certificate without
needing to perform additional DNS record lookups. It will typically
not be used for general DNSSEC validation of TLS endpoint names.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 28, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Requirements Notation . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
3. DNSSEC Authentication Chain Extension . . . . . . . . . . . . 3
3.1. Protocol . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. DNSSEC Authentication Chain Data . . . . . . . . . . . . 4
4. Construction of Serialized Authentication Chains . . . . . . 5
5. Caching and Regeneration of the Authentication Chain . . . . 6
6. Verification . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Trust Anchor Maintenance . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
11. Test Vectors . . . . . . . . . . . . . . . . . . . . . . . . 8
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
12.1. Normative References . . . . . . . . . . . . . . . . . . 8
12.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Pseudocode example . . . . . . . . . . . . . . . . . 10
Appendix B. Test vector . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Introduction
This draft describes a new TLS [RFC5246] extension for transport of a
DNS record serialized with the DNSSEC signatures [RFC4034] needed to
authenticate that record. The intent of this proposal is to allow
TLS clients to perform DANE authentication [RFC6698] of a TLS server
certificate without performing perform additional DNS record lookups
and incurring the associated latency penalty. It also provides the
ability to avoid potential problems with TLS clients being unable to
look up DANE records because of an interfering or broken middlebox on
the path between the endpoint and a DNS server. And lastly, it
allows a TLS client to validate DANE records itself without needing
access to a validating DNS resolver to which it has a secure
connection. It will typically not be used for general DNSSEC
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validation of endpoint names, but is more appropriate for validation
of DANE records such as TLSA, SMIMEA, etc.
This mechanism is useful for TLS applications that need to address
the problems described above, typically web browsers or VoIP and XMPP
services. It may not be relevant for many other applications. For
example, SMTP MTAs are usually located in data centers, may tolerate
extra DNS lookup latency, are on servers where it is easier to
provision a validating resolver, and are less likely to experience
traffic interference from misconfigured middleboxes. Hence DANE
authentication of SMTP MTAs [DANESMTP] is not likely to gain much
advantage from this mechanism.
The extension described here allows a TLS client to request in the
client hello message that the DNS validation chain be returned in the
(extended) server hello message. If the server is configured for
DANE authentication, then it performs the appropriate DNS queries,
builds the validation chain, and returns it to the client. The
server will usually use a previously cached authentication chain, but
it will need to rebuild it periodically as described in Section 5.
The client then authenticates the chain using a pre-configured trust
anchor.
This specification is based on Adam Langley's original proposal for
serializing DNSSEC authentication chains [AGL] and it incorporates
his ideas and some of his text. It modifies his approach by using
DNS wire formats and assumes that in implementation, the serialized
DNSSEC object will be prepared by a DNS-specific module and the
validation actions on serialized DNSSEC will also be carried out by a
DNS-specific module. An appendix (empty in the 00 version) provides
a Python code example of interfacing with a DNS-specific module.
3. DNSSEC Authentication Chain Extension
3.1. Protocol
A client MAY include an extension of type "dnssec_chain" in the
(extended) ClientHello. The "extension_data" field of this extension
MUST be empty.
Servers receiving a "dnssec_chain" extension in the client hello
SHOULD return a serialized authentication chain in the extended
ServerHello message, using the format described below. If a server
is unable to return a authentication chain, or does not wish to
return a authentication chain, it does not include a dnssec_chain
extension. As with all TLS extensions, if the server does not
support this extension it will not return any authentication chain.
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3.2. DNSSEC Authentication Chain Data
The "extension_data" field of the "dnssec_chain" extension represents
a sequence of DNS resource record sets, which provide a chain from
the DANE record being provided to a trust anchor chosen by the
server. The "extension_data" field MUST contain a DNSSEC
Authentication Chain encoded in the following form:
struct {
opaque rrset<0..2^16-1>;
opaque rrsig<0..2^16-1>;
} RRset
RRset AuthenticationChain<0..2^16-1>;
Each RRset in the authentication chain encodes an RRset along with a
signature on that RRset. The "rrsig" field contains the RDATA for
the RRSIG record, defined in Section 3.1 of RFC 4034 [RFC4034]. The
"rrset" field contains the covered resource records, in the format
defined in Section 3.1.8.1 of RFC 4034 [RFC4034]:
signature = sign(RRSIG_RDATA | RR(1) | RR(2)... )
RR(i) = owner | type | class | TTL | RDATA length | RDATA
The first RRset in the chain MUST contain the DANE records being
presented. The subsequent RRsets MUST be an sequence of DNSKEY and
DS RRsets, starting with a DNSKEY RRset. Each RRset MUST
authenticate the preceding RRset:
For a DNSKEY RRset, one of the covered DNSKEY RRs MUST be the
public key used to verify the previous RRset.
For a DS RRset, the set of key hashes MUST overlap with the
preceding set of DNSKEY records.
In addition, a DNSKEY RRset followed by a DS RRset MUST be self-
signed, in the sense that its RRSIG MUST verify under one of the keys
in the DNSKEY RRSET.
The final RRset in the authentication chain, representing the trust
anchor, SHOULD be omitted. In this case, the client MUST verify that
the key tag and owner name in the final RRSIG record correspond to a
trust anchor.
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For example, for an HTTPS server at www.example.com, where there are
zone cuts at "com." and "example.com.", we might get the following
RRsets:
. DNSKEY
com. DS
com. DNSKEY
example.com. DS
example.com. DNSKEY
_443._tcp.www.example.com. TLSA
Obviously, an authentication chain will be most compact and easiest
to verify if each RRset has a single record, i.e., if there is a
single DNSKEY RR and a single DS RR at each step. In addition, as
suggested above, keeping zone cuts to a minimum also reduces the
length of the authentication chain.
4. Construction of Serialized Authentication Chains
This section describes a possible procedure for the server to use to
build the serialized DNSSEC chain.
When the goal is to perform DANE authentication [RFC6698] of the
server's X.509 certificate, the DNS record to be serialized is a TLSA
record corresponding to the server's domain name.
The domain name of the server MUST be that included in the TLS Server
Name Indication extension [RFC6066] when present. If the Server Name
Indication extension is not present, or if the server does not
recognize the provided name and wishes to proceed with the handshake
rather than aborting the connection, the server uses the domain name
associated with the server IP address that the TLS connection arrives
on.
The TLSA record to be queried is constructed by prepending the _port
and _transport labels to the domain name as described in [RFC6698],
where port is the port number associated with the TLS server. The
transport is "tcp" for TLS servers, and "udp" for DTLS servers.
The components of the authentication chain are built by starting at
the trust anchor DNSKEY (usually expected to be the DNS root trust
anchor) and its corresponding RRSIG signature record, and then for
each intervening zone cut, adding the DS record and DNSKEY RRsets and
their RRSIGs, and finally the target TLSA record and RRSIG. (As
required above, however, the serialized authentication chain will
present the RRsets in the opposite order.)
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If the server acts as its own full iterative DNS resolver, it can
just build the chain as it performs normal iterative resolution of
the target record. If the server uses a recursive resolver, it
employs a slightly modified lookup algorithm, starting at the trust
anchor, prepending additional labels, and looking for NS, DS, and
DNSKEY records, until it reaches the target name.
In order to meet the formatting requirements above, the server must
perform some pre-processing on the resource records it receives. It
must first compute the uncompressed representation of the RRs,
removing DNS name compression [RFC1035] if present. It then extracts
the relevant fields from the resource records and assembles them into
an RRset.
5. Caching and Regeneration of the Authentication Chain
DNS records have Time To Live (TTL) parameters, and DNSSEC signatures
have validity periods (specifically signature expiration times).
After the TLS server constructs the serialized authentication chain,
it can cache and reuse it in multiple TLS connection handshakes.
However, it should keep track of the TTLs and signature validity
periods and requery the records and rebuild the authentication chain
as needed. A server implementation could carefully track these
parameters and requery the chain correspondingly. Alternatively, it
could be configured to rebuild the chain at some predefined periodic
intervals.
6. Verification
A TLS client making use of this specification, and which receives a
DNSSEC authentication chain extension from a server, SHOULD use this
information to perform DANE authentication of the server certificate.
In order to do this, it uses the mechanism specified by the DNSSEC
protocol [RFC4035]. This mechanism is sometimes implemented in a
DNSSEC validation engine or library.
If the record is correctly authenticated, the client then performs
DANE authentication according to the DANE TLS protocol [RFC6698].
7. Trust Anchor Maintenance
The trust anchor may change periodically, e.g. due to a key rollover
event by the operator of the initial zone. Managed key rollovers
typically use a process that can be tracked by verifiers allowing
them to automatically update their trust anchors, as described in
[RFC5011]. TLS clients using this specification are also expected to
use such a mechanism to keep their trust anchors updated. Some
operating systems may have a system-wide service to maintain and keep
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up-to-date the root trust anchor. It may be possible for the TLS
client application to simply reference that as its trust anchor,
periodically checking whether it has changed.
8. Security Considerations
The security considerations of the normatively referenced RFCs (1035,
4034, 4035, 5246, 6066, 6698) all pertain to this extension. As
mentioned above, there are particular security pitfalls in creating
and using this serialization because it has very different temporal
qualities from the usual certificate that would be validated. So the
requirements in Section 6 (Section 5) for not caching and for
maintaining very good clock synchronization on the client are quite
important for avoiding risks of replay or of use of revoked
certificates. Other residual risks of this specification include
locating the validation function in the server rather than in the
client. This might seem reasonable on the face of it, but because
the DNSSEC serialization is sent in the clear in the client hello, it
could be tampered with and the certificate fingerprint or full
certificate (depending on the mode) should not be used without
performing the DNSSEC validation (in the DNS-specific module.
DNSSEC signatures have validity periods defined by an inception and
expiration time. TLS clients need roughly accurate time in order to
properly authenticate these signatures. This could be achieved by
running a time synchronization protocol like NTP [RFC5905] or SNTP
[RFC4330], which are already widely used today.
9. IANA Considerations
This extension requires the registration of a new value in the TLS
ExtensionsType registry. The value requested from IANA is 53. If
the draft is adopted by the WG, the authors expect to make an early
allocation request as specified in [RFC7120].
10. Acknowledgments
Many thanks to Adam Langley for laying the groundwork for this
extension. The original idea is his but our acknowledgment in no way
implies his endorsement. This document also benefited from
discussions with and review from the following people: Allison
Mankin, Duane Wessels, Willem Toorop, Jeff Hodges, and Gowri
Visweswaran.
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11. Test Vectors
[TO BE ADDED LATER. THE ORIGINAL CONTENT WAS OBSOLETE.]
12. References
12.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, August 2012.
12.2. Informative References
[RFC4330] Mills, D., "Simple Network Time Protocol (SNTP) Version 4
for IPv4, IPv6 and OSI", RFC 4330, January 2006.
[RFC5011] StJohns, M., "Automated Updates of DNS Security (DNSSEC)
Trust Anchors", STD 74, RFC 5011, September 2007.
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
[RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
Points", BCP 100, RFC 7120, January 2014.
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[AGL] Langley, A., "Serializing DNS Records with DNSSEC
Authentication", <https://tools.ietf.org/id/draft-agl-
dane-serializechain-01.txt>.
[DANESMTP]
Dukhovni, V. and W. Hardaker, "SMTP Security via
opportunistic DANE TLS", <https://tools.ietf.org/html/
draft-ietf-dane-smtp-with-dane-19>.
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Appendix A. Pseudocode example
[code goes here]
Appendix B. Test vector
[data go here]
Authors' Addresses
Melinda Shore
No Mountain Software
EMail: melinda.shore@nomountain.net
Richard Barnes
Mozilla
EMail: rlb@ipv.sx
Shumon Huque
Verisign Labs
EMail: shuque@verisign.com
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